1. Technical Field of the Invention
The present invention relates to an image forming apparatus having a function of correcting gradation characteristics of an image forming section on the basis of an internal sensor.
2. Description of the Related Arts
Conventionally, there has been known an image forming apparatus such as a color copying machine or a color printer, having a sensor inside its machinery in order to output an image having a good gradation, with a function of reading patch patterns having a plurality of gradation tones formed inside the image forming section by using the sensor, generating a correction table so as to achieve a desired value from the obtained value, and correcting the gradation tones by using the correction table (for example, refer to Japanese Unexamined Patent Publication (Kokai) No. 2001-251510 and Japanese Unexamined Patent Publication No. Hei 6-197873).
In the above, the sensor practically outputs a reflectance or the like as an output value. Then, the image forming apparatus converts the sensor output value to, for example, an XYZ value by using a certain conversion curve to use it as a sensor read value for a gradation correction. The conversion curve has been obtained by forming and outputting the same image as the above patch patterns on transfer paper and calculating a conversion coefficient with carrying out a regression calculation using a result of color measuring with a calorimeter and a reflectance of the sensor. The coefficient is a fixed value.
The sensor reading characteristics, however, may change with time due to a change in sensor characteristics such as a consumption or a change of toner or any other material, a dirt on the sensor, a displacement or an inclination of the sensor, or the like, thereby causing the conversion coefficient to be unsuitable and resulting in an unstable output result even after the gradation correction has been made using the sensor.